Ive given up on that. Videos I've watched show too many different ways. I've been checking wings where they say hens have basically 2 layers with longer and shorter, roo's have even single layer. There several I've watched that show this method:
I've tried a few too. I was wrong. The people who do this as a job, must know a secret.
The guy at TSC and I measured wings and something else but he said he was clueless too. I was wanting a roo so I bought 6 straight runs and got 2 that I had for years. They died last yr and a neighbor gave me her spare.